A number of optical communication devices and systems are configured for use with optical wavelength division multiplexing (WDM). In WDM, several information signals can be transmitted over a single optical fiber. Each information signal is used to modulate a different wavelength carrier signal or channel. Many operations in such systems require multiplexing or demultiplexing the signals. Multiplexing generally involves combining multiple channels into a single WDM signal. Demultiplexing generally involves extracting or separating individual channels, e.g. for subsequent processing such as routing to desired pathways and/or outputting to output ports. Each channel occupies or is contained within a predefined frequency range. In many (but not all) systems the frequency ranges defining the boundaries for the channels are equal in bandwidth, evenly spaced and contiguous. Since the total bandwidth for a WDM signal cannot be significantly smaller than the sum of the bandwidths of the component channels, for a given bandwidth WDM signal, the number of channels that can be defined (and thus the number of separate information signals that can be carried) increases as the bandwidth of the channels decreases. Thus it is not surprising that systems have tended toward increasingly-smaller channel spacing to increase the number of optical channels. Many systems would, accordingly, benefit from optical devices which can successfully accommodate channel bandwidths of, for example, 100 GHz, 50 GHz or even less.
Unfortunately, as the number of channels is increased, the channel spacing is decreased so that functions such as demultiplexing/multiplexing have become increasingly difficult. In addition to the difficulty of constructing devices with sufficient accuracy to, even theoretically, demultiplex such narrow-bandwidth channels, and without wishing to be bound by any theory, it is believed narrow-bandwidth demultiplexing is particularly susceptible to factors such as wavelength drift and/or channel cross-talk.
Accordingly, it would be useful to provide a demultiplexer/multiplexer which can provide useful results with respect to narrow-bandwidth channels such as 100 GHZ, 50 GHz channels or smaller, preferably while being relatively tolerant of wavelength drift and/or providing relatively low channel cross-talk.
It is, in general, believed possible to construct devices which permit channels to be controllably provided to desired output ports (i.e. routers) and/or permit a new information signal to replace an existing information signal in a channel of a WDM (i.e. an add/drop device). It is believed some such devices involve active (i.e. controlled) channel separation in which binary or other control signals function to achieve the desired routing or add/drop configuration. It is believed that many such systems are configured such that a change in the control signals (i.e. a change in routing or add/drop function) affects (e.g. interrupts or "touches") all or substantially all of the channels in the WDM signal. While this may be acceptable for some applications, it is believed other applications (such as asynchronous, continuous, real time and/or time critical signals) would be more readily served by devices which permitted re-routing or add/drop changes of only desired channels while leaving other channels in a WDM signal substantially unchanged or untouched.
Accordingly, it would be useful to provide a demultiplexer/multiplexer, and associated devices such as add/drop devices, routers, and the like, particularly narrow-bandwidth channel systems, in which the multiplex /demultiplex function was substantially passive and/or in which channels not being changed by a change in routing or add/drop function would be substantially unaffected or untouched.